S12.23: PassthroughBuffer error path completeness

[DavidCozens]

Parent epic: #31

Scope

Pin the residual error path on SolidSyslogPassthroughBuffer left over from S12.07's audit of IGNORE_TEST placeholders. PMQB's equivalent block was deleted in #443 once every referent was either pinned by a real test or explicitly ruled out of scope; the PassthroughBuffer block at Tests/SolidSyslogPassthroughBufferTest.cpp:102 is the smaller sibling — same shape, no syscalls.

In scope — production

• _Create(NULL sender) → release the acquired pool slot, return the shared SolidSyslogNullBuffer fallback, emit a new POSIXMESSAGEQUEUEBUFFER_ERROR_NULL_SENDER-equivalent error code (PASSTHROUGHBUFFER_ERROR_NULL_SENDER). Currently the NULL sender is stored unchecked and the first subsequent Write dispatches SolidSyslogSender_Send(NULL, ...) → crash.

In scope — tests only (production already safe)

• Destroy(NULL) — routes via PassthroughBuffer_IndexFromHandle → POOL_SIZE → IndexIsValid=false → UNKNOWN_DESTROY warning by the same fallthrough that PMQB exercises. Pin it with a literal NULL test.
• Use-after-Destroy via NullBuffer vtable patch — already pinned by UseAfterDestroyIsCrashSafeViaNullBufferVtable. Just delete the IGNORE_TEST placeholder once the slot-release/NULL-sender slice lands.

Out of scope (deliberate)

• Write(NULL buffer, ...) — routes through SolidSyslogBuffer_Write in Core/Source/SolidSyslogBuffer.c, which intentionally doesn't guard. Buffer-NULL is a SolidSyslog_Log-entry concern (already on E12's "Outstanding — NULL guards on public entry points" list), not a PassthroughBuffer concern.

Error code growth

PASSTHROUGHBUFFER_ERROR_POOL_EXHAUSTED      (existing)
PASSTHROUGHBUFFER_ERROR_UNKNOWN_DESTROY     (existing)
PASSTHROUGHBUFFER_ERROR_NULL_SENDER         (new)
PASSTHROUGHBUFFER_ERROR_MAX

Plus a message string in Core/Source/SolidSyslogPassthroughBufferMessages.c.

Slice walk

1. NULL-sender + slot release. Test: _Create(NULL) returns a fallback handle distinct from any pool slot, NULL_SENDER ERROR emitted, slot released (next _Create reuses it). Production: guard at _Create entry, free the slot if Initialise was already entered, return SolidSyslogNullBuffer_Get().
2. Destroy(NULL) pin. Test only — production already safe by fallthrough.
3. Delete IGNORE_TEST(SolidSyslogPassthroughBuffer, HappyPathOnly) at Tests/SolidSyslogPassthroughBufferTest.cpp:102. All referents pinned or explicitly out of scope.

Reference

Spawned by S12.07 (#118) — PR #443's audit identified the last IGNORE_TEST in the codebase. The two stories' patterns mirror each other; the PMQB experience suggests this is roughly half-day-ish work given there are no syscalls and no fake to build.